Ferrofluids are colloidal systems containing magnetic particles, typically having a diameter of the order of 10 nm, suspended in a liquid carrier. The difference between ferrofluids and other well-known colloids is that ferrofluids specifically utilize particles which possess magnetic properties, whereas other colloids are comprised of nonmagnetic particles. Commercial ferrofluids are generally comprised of magnetite or mixed ferrite particles, although it is possible to use particles of other magnetic materials, such as iron, cobalt, nickel, chromium dioxide, iron nitride or magnetic alloys of these materials. In general, the ferrofluid is colloidally stable and has a relatively low viscosity.
In order to form a stable colloid, the particle size should be in the range of 10 nm. In practice, the particles are generally spherical in shape and are small enough to form a single magnetic domain representing a tiny permanent magnet with an associated north and a south pole. The particles are further coated with layers of surfactant to prevent agglomeration caused by magnetic and Van der Waals attractive forces. It is also possible to form a stable colloid by using either positive or negative electrical charges to keep the particles separated; these formulations are referred to as "ionic ferrofluids". The surfactant used in each type of ferrofluid is specific to the carrier because the surfactant must be chemically compatible with the carrier. The particles may be coated either with a single or double surfactant layers and may be either cationic, anionic or non-ionic in nature.
A typical ferrofluid may consist of the following volume fractions: 4% particles, 8% surfactant and 88% liquid carrier. Ferrofluids are characterized by the liquid carrier in which the particles are suspended because it is the dominant component. For example, a water-based ferrofluid is a stable suspension of magnetic particles in water, whereas an oil-based ferrofluid is a stable suspension of magnetic particles in an oil (such as a hydrocarbon, an ester, a fluorocarbon, a silicone oil or polyphenyl ether, etc.) The physical properties of a ferrofluid are also based on the selection of the liquid carrier since it is the majority component. In addition, as mentioned above, the surfactants for water- and oil-based ferrofluids are different.
Magnetic colloids can be viewed as "liquid magnets" which can be manipulated and positioned with a magnetic field to provide sealing and increase heat transfer rates. The commercial applications of ferrofluids include sealing, damping, heat transfer, noise control, material separation, sensing and parts inspection. They are used in such diverse products as exclusion seals, loudspeakers and stepper motors. The industries in which ferrofluid-based products are typically used are: the semiconductor, computer, aerospace, oil prospecting and mining industries.
Known manufacturing processes for producing ferrofluids characteristically begin by producing suitably-sized particles of a ferrous material, such as Fe.sub.3 O.sub.4 (magnetite). Magnetite particles in a subdomain size (e.g., about 10 nm) necessary for use in a ferrofluid, are not commercially available. Consequently, two methods are used to produce suitably sized particles: ball mill grinding and chemical precipitation. These methods are described in detail in a book entitled Ferrohydrodynamics by R. E. Rosensweig; Cambridge University Press and a book entitled Magnetic Fluid Handbook and Applications Handbook, editor B. Berkovoski, Begell House, Inc., New York (1996).
A typical ball mill grinding process starts with commercially-available magnetic powder, such as magnetite powder, in which the particles are of "micron size", e.g. about 0.15 to 0.3 microns (i.e. about 150 nm to 300 nm). The commercially-available magnetite particles are then ground to reduce their size about 90%, to about 10 nm. A typical ball milling process is described in the U.S. Pat. No. 3,917,538. In this process, proper amounts of magnetite powder, surfactant and a solvent are placed in a stainless steel milling jar about 40% filled with grinding media, such as 1/4" carbon steel balls. For grinding to be effective, the viscosity of the solvent should be low. In a water-based ferrofluid, the water carrier is of low viscosity and consequently, it can be used as the solvent in the grinding process. In oil-based ferrofluid, the carrier liquid often has a relatively high viscosity. Consequently, a low molecular weigh solvent is often added to the oil carrier during the grinding process to reduce the viscosity. This solvent is subsequently removed by evaporation to increase the saturation magnetization in the final ferrofluid.
The rolling action of the mill causes the media to impact repeatedly the coarse magnetite breaking it into subdomain size particles and coating some of the particles at the same time with the surfactant. Because the milling media generate a relatively low shear energy, a conventional ball milling operation takes anywhere from two to six weeks to complete and the dispersion quality is poor. The colloid formed by this process generally includes uncoated particles and large aggregates and thus requires a subsequent refinement in which undesirable particles and aggregates are removed.
Moreover, the finished product often has a high viscosity due to the presence of small particles produced during the grinding process. These small particles are further known to degrade the thermal stability of the fluid. It is also believed that the process may only reduce large aggregates of useful small particles which are initially present in commercial magnetite powder. The yield is poor, preparation times are long and the associated costs are high so that the ball milling method is generally not considered to be suitable for a large scale production of commercial ferrofluids.
Magnetite particles can also be produced by chemical precipitation processes. Generally, in such processes, magnetite particles are produced by mixing ferrous and ferric salt solutions in the presence of an alkaline medium. The resultant particles are then coated with surfactant. Both water-based and oil-based ferrofluids can be produced by means of this technique. For example, U.S. Pat. No. 5,240,626 discloses the synthesis of a water-based ferrofluid in which nanosize magnetite particles are coated with a single carboxyl-functional polymer surfactant. Two separate surfactant coatings are used for magnetite particles in aqueous phase in U.S. Pat. No. 4,094,804. Lignosulphonate, a byproduct of wood pulping, was used to prepare an inexpensive water-based colloid by chemically precipitating magnetite microcrystals as disclosed in U.S. Pat. No. 4,110,208.
However, ferrofluid produced by these processes typically requires extensive processing after the particles have been generated to confine the particle sizes to an acceptable range. The resulting ferrofluid also has a high viscosity and its magnetization is low. Consequently, this fluid is not suitable for many practical applications. In addition, chemical precipitation requires the use of several chemicals, extensive processing operations such as the washing of particles, controlled heating for the attachment of surfactant and magnetic separation to separate phases. There is also chemical waste generated during the process. The colloids thus prepared are expensive.
Although many processes for manufacturing ferrofluids are known, even after 25 years of their existence, these fluids are still speciality products, produced in small amounts with high manufacturing costs. Therefore, ferrofluids presently are not commercial feasible for high volume applications, such as power transformers and material separators.
Accordingly, there is a need for a process which produces an inexpensive ferrofluid which can quickly be manufactured in large volumes. It is also desirable that such a fluid be water-based, because water-based fluids involve the use of minimum chemical ingredients and can be very low cost since the water carrier is inexpensive. It is further desirable that the ferrofluid be produced with a process that generates no waste and is not labor intensive.